In A Nutshell

Mt. Rainier in Washington State could potentially cause a massive disaster without even erupting. The mountain’s interior is soft and muddy due to hydrothermal forces. Part or all of Rainier may collapse in on itself, creating massive mudslides called lahars which may reach the Puget Sound. Such a collapse could occur without the typical warning signs that would precede a regular volcanic eruption.

The Whole Bushel

Mt. Rainier in western Washington State is considered to be one of the most dangerous volcanoes in the United States. While it is certainly still capable of erupting in a traditional manner, Mt. Rainier could cause a major catastrophe without doing so. At any moment, the mountain could collapse in upon itself, causing gigantic mudflows known as lahars.

Mt. Rainier is structurally unstable. It has an active hydrothermal system which bombards its interior with acidic steam, leaving much of the subsurface rock with the consistency of soft clay. Eventually, the surface will crumble due to this muddy foundation. To compound the issue, Mt. Rainier has 93 square kilometers (36 sq mi) of glaciers covering its slopes, averaging 30 meters (100 ft) thick. These glaciers erode cracks into Mt. Rainier’s surface, further weakening it, as well as trickling water down into the acidic sludge below.

Mt. Rainier has produced at least 55 lahars in the last 10,000 years. While a number have been caused by traditional volcanic eruptions, the largest have been caused by the collapse of parts of the mountain’s surface.

Five thousand six hundred years ago, the top of the mountain collapsed inward and created one of the largest lahars known to have ever occurred. This event, known as the Osceola Mudflow, covered 550 square kilometers (212 sq mi) of land in mud and reached the Puget Sound. The collapse that originated the mudflow left Mt. Rainier as only a horseshoe-shaped crater much like Mt. St. Helens after its infamous 1980 eruption. This crater, called the Osceola Crater, was mostly filled in by volcanic eruptions by 2,200 years ago, although even today the top of Mt. Rainier is noticeably blunted-looking. If the Osceola Mudflow happened today, it would reach Kent, a suburb in southern Seattle, as well as plowing straight through Tacoma and into Commencement Bay. Many smaller towns closer to Mt. Rainier would be completely buried.

Other mudflows have occurred. Another lahar, the Round Pass Mudflow, shot out of a collapse on the west side of Osceola Crater roughly 2,500 years ago and covered parts of the Puyallup River with 30 meters (100 ft) of mud. The Electron Mudflow, the most recent major lahar, happened only 500 years ago, and covered the modern-day site of the town of Electron with about the same depth of mud as the Round Pass Mudflow. Smaller collapses causing avalanches which can at least threaten the Mt. Rainier National Park area occur roughly once a century.

Authorities are quite aware of the danger a future collapse could pose, as over 150,000 people live in areas where lahars have hit before. Dauntingly, the collapse of all or part of Mt. Rainier could occur without the typical warning signs that a volcanic eruption would present. Special sensors designed to detect lahars have been placed in the most danger-prone areas. Monitoring computers are programmed to trigger urgent alarms if these sensors should fall silent, exactly what would happen if a lahar swept them away. Lahars from the mountain could hit the nearest communities in as little as a half hour. Should the sirens go off, residents are instructed to abandon everything and climb to higher ground, as lahars are governed by gravity much like water from a failing dam.

Mt. Rainier, while a beautiful sight, is well known to be very dangerous. The question of an eruption has never been ‘if’ but always ‘when’. This kind of event is something I’d never heard of, good informative article.

Clyde Barrow

The whole Cascade range is dangerous. Weed, California is right in the damage path of previous lahars from Mt. Shasta. Mt. Hood in Oregon is also said to be near the end of it’s ‘cycle’ time for another eruption.

Just like you mentioned, it’s like an earthquake in a fault zone. It’s not if, but when.

Dave Stevens

What is worrying is that I can find no map of the location, number, or testing procedures of the lahar sensors. They were put in place by the USGS and the system was turned over to Pierce County in 2004. The USGS recommended expanding the system which, at that time I believe consisted of 5 sensors. Another problem is that there is no record of evacuation simulations. In Indonesia, there are physical evacuation drills in high risk regions. The Carbon and Puyallup River plains continue to be an extremely high risk situation that is not taken seriously enough by either the residents of or administrators of that region.